A variety of gas-phase reaction methods have been developed for the purpose of smoothing surfaces of electronic devices and the like and have been put to practical use. For example, a substrate surface smoothing method disclosed in Patent literature 1 smoothes a substrate surface by sputtering using monatomic or monomolecular ions of Ar (argon) gas directed onto the substrate surface at a low angle.
Recently, solid surface smoothing methods using a gas cluster ion beam have been attracting attention because they can reduce surface roughness greatly without damaging the surface badly. For example, Patent literature 2 discloses a method of reducing surface roughness by irradiating a solid surface with a gas cluster ion beam. In this method, gas cluster ions directed onto the workpiece (solid) dissociate when they collide with the workpiece. In this process, multibody collisions occur between atoms or molecules forming the cluster and atoms or molecules forming the workpiece, causing noticeable motion in a lateral direction with respect to the workpiece surface (solid surface). As a result, the workpiece surface is cut laterally. This phenomenon is called lateral sputtering. The motion of particles in a lateral direction with respect to the workpiece surface mainly cuts projecting portions from the surface, performing ultraprecise polishing to produce a smooth surface at the atomic level.
In the gas cluster ion beam, an ion has a lower energy than that in normal ion etching. In other words, a single atom or molecule forming the cluster has a lower energy. This enables ultraprecise polishing as needed, without damaging the workpiece surface. One advantage of solid surface smoothing using a gas cluster ion beam is that the damage to the workpiece surface is less than that caused by ion etching, which is disclosed in Patent literature 1.
In solid surface smoothing using a gas cluster ion beam, it is generally recognized that the workpiece surface should be irradiated with the cluster ion beam at approximately right angles to the workpiece surface. This angle makes it possible to make maximum use of the effect of surface smoothing by lateral sputtering described above.
Patent literature 2 discloses that a curved surface or the like may be irradiated in an oblique direction, depending on the surface condition, but the effect of such oblique irradiation is not mentioned. Therefore, Patent literature 2 implies that approximately perpendicular irradiation of the solid surface is the most efficient for surface smoothing.
Patent literature 3 discloses another example of solid surface smoothing by using a gas cluster ion beam. However, Patent literature 3 does not describe the relationship between surface smoothing and the angle formed by the gas cluster ion beam and the solid surface. Since the description indicates that the lateral sputtering effect is used, it is inferred that, like Patent literature 2, Patent literature 3 shows data for perpendicular irradiation.
Non-patent literature 1 also includes a report of solid surface smoothing by gas cluster ion beam irradiation. In that literature, Toyoda and others report that surface roughness is reduced by irradiating the surface of materials such as Cu, SiC, and GaN with Ar cluster ions. The surface was irradiated with the gas cluster ion beam at approximately right angles.
Non-patent literature 2 describes variations in the roughness of a solid surface when the solid surface is irradiated with a gas cluster ion beam at a variety of irradiation angles. When the solid surface is irradiated at right angles, the irradiation angle is expressed as 90 degrees (the symbol ° will be used hereafter to express an angle). When the surface is irradiated laterally, the irradiation angle is expressed as 0°. The literature discloses that the sputtering rate, which indicates a speed at which the surface is etched, is maximized by perpendicular irradiation and that the etching rate decreases as the irradiation angle decreases. The relationship between surface roughness and irradiation angle was observed through experiments at different irradiation angles of 90°, 75°, 60°, 45°, and 30°. According to the literature, the surface roughness increased with a decrease in irradiation angle. No experiments were made at irradiation angles below 30°. It may have been thought that such experiments would be useless.
It was recently found that the roughness of a solid surface decreased greatly by reducing the angle of gas cluster ion beam irradiation with respect to the solid surface to below 30° (refer to Patent literature 4). This technology uses an oblique irradiation effect, and the smoothing mechanism differs from that in the conventional lateral sputtering. Patent literature 4 describes the use of a plurality of irradiation angles in irradiation of the solid surface with the gas cluster ion beam. The irradiation is performed at different angles in succession.
Patent literature 1: Japanese Patent Application Laid Open No. H7-58089
Patent literature 2: Japanese Patent Application Laid Open No. H8-120470
Patent literature 3: Japanese Patent Application Laid Open No. H8-293483
Patent literature 4: WO2005/031838
Non-patent literature 1: Jpn. J. Appl. Phys., Vol. 41 (2002), pp. 4287-4290
Non-patent literature 2: Materials Science and Engineering R 34 (2001), pp. 231-295